In the last decade, fiber optic transmission lines have come into widespread use in the telecommunications industry. Fiber optics offer several advantages as opposed to copper wire including higher bandwidth, better resistance to harsh conditions, and less signal attenuation.
In the process for manufacturing fiber optics, the fiber, after being drawn from a heated preform is coated with a photocurable coating, and is then exposed to ultraviolet radiation to cure the coating.
The source which produces the ultraviolet radiation typically is comprised of at least one microwave powered or arc lamp discharge lamp positioned so as to irradiate the fiber as it is run through a housing. These lamps unavoidably produce such a large amount of heat that it is necessary to cool the housing and/or the lamp bulb with forced air or a liquid coolant. A transparent chamber defines a space in the housing through which the fiber is passed. The chamber serves to contain an atmosphere which is inert with respect to the coating on the fiber, and also shields the fiber from the vigorously circulated lamp coolant.
A problem which is encountered with arrangements of the type described above is that heat or infrared radiation generated by the lamp causes coating material to be evaporated from the fiber, which is deposited on the wall of the transparent chamber, thus darkening it.
Japanese laid open Patent 55-152567 discloses an ultraviolet light source for curing a coating on a fiber. The light source includes a quartz tube as a chamber which contains an inert atmosphere of nitrogen gas through which the fiber is passed. As described above, an apparatus of this type has the defect that coating material evaporated from the fiber tends to deposit on the inside of the tube, and thereby reduces the amount of light reaching the fiber. Industrial practice is to replace the tube after each shift but there remains the problem that the light reaching the fiber diminishes as the line is run. There is also a problem that the temperature of the fiber as it is running through the light source is too high and not easily controlled. This is caused by the bombardment of the fiber by infrared radiation which is unavoidably produced by arc lamps and microwave lamps used in fiber curing apparatus. The problem of excessive heating aggravates the problem of light blocking deposits on the tube since the rate of evaporation of coating from the fiber increases with temperature.
U.S. Pat. No. 4,636,405 to Mensah et al. discloses the use of a double walled tube through which an infrared absorbing fluid is circulated. The circulating fluid is provided to lower the temperature in the chamber. However, coating material evaporated from the fiber is not prevented from settling on the inside of the tube and diminishing the light that reaches the fiber. Additionally, the double walled tube is a custom made glass piece, which is relatively expensive.
The problem of fiber overheating becomes worse at low line speeds. In this case the dwell time of the fiber in the chamber is longer, and thus the coating which it bears is subject to more heating, which causes more evaporation and deposits on the chamber wall.